Awesome Fireball Event in central Russia

2012 DA14 may not be on a collision course with Earth later today but a smaller asteroid was. A major fireball (and most likely also a meteorite dropping event) occurred over the city of Chelyabinsk, Russia. Chelyabinsk is a city of 1+ million people located just to the East of the Ural Mountains and just north of the Russia-Kazakhstan border.

The fireball that occurred there this morning appeared brighter than the Sun and produced a sonic boom that shattered windows causing flying glass-induced injuries to hundreds of people. A large building in town also seems to have been damaged. Though it is still uncertain if this was due to a large meteorite or the sonic boom.

An event like this happening only hours before the close flyby of the ~45-meter in diameter asteroid 2012 DA14, begs the question of whether the two are linked. It is probably unlikely that the Chelyabinsk fireball and 2012 DA14 are related. Luckily there are so many great videos of the fireball that an accurate orbit for the asteroid that caused the fireball should be easily determined.

[Update: 2012 DA14 and the Russian fireball can not be related. The radiant (the region of the sky that a DA14 or a piece of DA14 would appear to come from) of DA14 is at the very far southern declination of -81 degrees. This is the reason why DA14 is only visible from the southern hemisphere as it approaches Earth. A radiant that far south could not produce a fireball over Russia which is in the northern hemisphere. Any pieces of DA14 would only be able to impact Earth over the southern hemisphere or a few degrees north of the Equator. The fact that the Russian fireball and the 2012 DA14 close approach are happening on the same day is just a coincidence.]

Up-to-date information can be found at RT, here and here, and RMNB.

Many videos have been posted. The first 2 show the fireball itself. The last 2 are videos of the resulting contrail. What is very impressive about the last two is that the videos also caught the sonic boom. In one of the videos you can hear glass shattering in the background. Simply awesome…

Recent Discoveries – Sept 17-24, 2010

Catching up on the last week in comet and asteroid discoveries… A week ago new NEA announcements were coming left and right but the flood completely stopped a few days before Full Moon. Most surveys take a break for a few days around Full Moon since the bright sky is really not conducive for finding much of anything.

Last week saw one and probably two comet discoveries. C/2010 S1 (LINEAR) is a large perihelion (q = 4.4 AU) long-period comet. Currently 17th magnitude and 8.5 AU from the Sun (almost the distance of Saturn), it should brighten to magnitude 12-13 near perihelion in the summer of 2013. This comet marks the 44th (non-SOHO-STEREO) comet discovery of the year and LINEAR’s 199th comet find.

2010 BK118 was first seen back in January by the WISE (Wide Infrared Space Explorer) spacecraft. WISE is conducting a survey of the sky at 4 infrared wavelengths. Though not specifically designed to find asteroids/comets, the spacecraft observes at wavelengths optimal for finding these types of objects. Though not identified as anything special back in January, it was independently rediscovered by LINEAR last week. After a few days of observation, January’s WISE object and this month’s LINEAR object were linked as one and the same. So far there have been no reports of cometary activity even though the orbit is very cometary (long-period type). If it is truly a extinct or dormant comet than it is very large for a comet nucleus at H = 10.2 and a diameter of ~60 km. I’d be very surprised if this object doesn’t turn out to be an actual comet when larger telescopes are pointed its way.

2010 SW3 passed within 0.0058 AU of Earth on Sept. 10. This distance equals 540,000 miles, 860,000 km or 2.1 Lunar Distances. The 10 to 30 meter in diameter rock was discovered by the Mount Lemmon Survey eight days after close approach.

Asteroid    Type     MOID     a     e     i     H  Mag  Discoverer      MPEC
2010 SE12   Amor    0.115   1.36  0.18   9.3  24.3  20  Spacewatch      2010-S34
2010 SD12   Amor    0.076   1.80  0.41  23.6  23.8  20  Mount Lemmon    2010-S33
2010 SA12   Amor    0.147   2.25  0.50   6.2  20.8  20  Mount Lemmon    2010-S32
2010 SX11   Apollo  0.026   1.16  0.25   5.3  24.8  20  Spacewatch      2010-S30
2010 SZ3    Apollo  0.014   1.18  0.14   2.0  28.3  20  Mount Lemmon    2010-S26
2010 SY3    Amor    0.224   2.08  0.41   6.0  22.8  21  Mount Lemmon    2010-S25
2010 SX3    Amor    0.066   1.59  0.33   8.0  24.9  20  Mount Lemmon    2010-S24
2010 SW3    Apollo  0.0005  1.62  0.40   1.6  26.6  20  Mount Lemmon    2010-S23
2010 SV3    Apollo  0.056   1.51  0.52   6.2  20.5  19  Catalina        2010-S21
2010 ST3    Apollo  0.040   2.06  0.53   3.8  25.1  21  PanSTARRS       2010-S20
2010 SS3    Amor    0.393   1.61  0.24  26.9  20.8  21  Mount Lemmon    2010-S19
2010 SR3    Amor    0.187   1.74  0.37  12.2  21.8  21  Mount Lemmon    2010-S18
2010 SP3    Apollo  0.004   1.98  0.63   0.4  24.6  18  Catalina        2010-S16
2010 RG137  Amor    0.105   2.36  0.55   9.9  21.0  19  Mount Lemmon    2010-S15

Comet       Type       T        q     a     e     i  Mag  Period        MPEC 
C/2010 S1 (LINEAR)
            LPC    2013-05-09  4.41       1.0  126.9  17                2010-S41
2010 BK118 (discovered by WISE, rediscovered by LINEAR)
            ECC    2010-01-01  6.12 293.7 0.98 143.9  19  5030          2010-S26

Type
Aten -  Earth crossing with semi-major axis (avg distance from Sun) < 1 AU
Apollo - Earth crossing with semi-major axis (avg distance from Sun) > 1 AU
Amor - non-Earth crossing with perihelion distance < 1.3 AU
JFC - Jupiter family comet
HFC - Halley family comet
LPC - Long-period comet
MBC - Main belt comet
ECC - Suspected extinct or dormant (or just unrecognized) comet
T - Date of Perihelion
MOID - Minimum Orbit Intercept Distance, minimum distance between asteroid and Earth's orbit
a - semi-major axis, average distance from Sun in AU (1 AU = 93 million miles)
e - eccentricity
i - inclination
H - absolute magnitude
Mag - magnitude at discovery
Discoverer - survey or person who discovered the object
MPEC - Minor Planet Electronic Circular, the discovery announcement


				

Recent Discoveries – Sept 13, 2010

Two days ago 10 NEAs were announced, yesterday saw only a single announcement. The reason… most asteroid surveys are located in the southwest US (Arizona and New Mexico) so a bout of clouds over this part of the country will put a major dent in the number of discoveries. Hopefully the discoveries will ramp back up again tonight.

Yesterday’s sole announcement was larger than most recent discoveries. Nowadays most of the larger NEAs have been found. 2010 RO82 is probably somewhere between 3/4 of a km and 2.5 km across. Chances are an object this big and bright has been seen before though it may not have been recognized as an NEA. Though it is an NEA it really doesn’t come very close to Earth (MOID of 0.163 AU).

Asteroid    Type     MOID     a     e     i     H  Mag  Discoverer      MPEC
2010 RO82   Amor    0.163   2.47  0.60  18.0  16.9  19  Siding Spring   2010-R107

Comet       Type     MOID     q     a     e     i  Mag  Discoverer      MPEC
None

Type
Aten -  Earth crossing with semi-major axis (avg distance from Sun) < 1 AU
Apollo - Earth crossing with semi-major axis (avg distance from Sun) > 1 AU
Amor - non-Earth crossing with perihelion distance < 1.3 AU
JFC - Jupiter family comet
HFC - Halley family comet
LPC - Long-period comet
MBC - Main belt comet
MOID - Minimum Orbit Intercept Distance, minimum distance between asteroid and Earth's orbit
a - semi-major axis, average distance from Sun in AU (1 AU = 93 million miles)
e - eccentricity
i - inclination
H - absolute magnitude
Mag - magnitude at discovery
Discoverer - survey or person who discovered the object
MPEC - Minor Planet Electronic Circular, the discovery announcement


				

Hayabusa Returns to Earth

At 1400 hours UT on Sunday June 13 a brilliant fireball will appear over Australia. Unlike most fireballs this one won’t be due to a small asteroid but a returning man-made spacecraft. The Japanese Hayabusa (originally called MUSES-C for Mu Space Engineering Spacecraft C) is returning home after a harrowing 7 year mission to a small near-Earth asteroid and back. Harrowing in that almost anything that could go wrong did go wrong. Yet, if all goes well Hayabusa will be the first spacecraft to return samples from an asteroid.

[NOTE: A team from NASA will observe and study the resulting fireball from a NASA research DC-8 aircraft. The airborne team will attempt to broadcast video of the even live. Live video will be shown at 13:45-13:55 UT (9:45-9:55 am EDT).]

Sketch of Hayabusa spacecraft hovering over the surface of asteroid Itokawa during sample collection. Credit: Wikipedia public domain.

Launched in May of 2003, Hayabusa spent just over 2 years traveling to the half-km in diameter Earth-crossing asteroid (25143) Itokawa. In November of 2005 two sample attempts were made. The plan was for Hayabusa to hover a few meters above the asteroid’s surface, fire a small projectile into the surface, and collect any material that was thrown upwards. Due to spacecraft hardware and software control failures mission operators are unsure if any samples were collected. In fact, no one is completely sure what happened when Hayabusa reached the asteroid’s surface due to a loss of contact.

Due to the great work of the mission team, the hobbled spacecraft was coaxed back to Earth in spite of losing its reaction wheels, reaction control system (fine thrusters), and some of its ion thrusters (main propulsion). Regardless of whether Hayabusa successfully lands in Australia on Sunday or whether or not it contains any samples from Itokawa, the mission was highly successful and produced some great data on a small near-Earth asteroid.

Image of asteroid Itokawa by the Japanese Hayabusa spacecraft. Credut: JAXA.

2010 KQ – a very small, very close asteroid

Small asteroids buzz past the Earth every day. The great majority of them pass by sight unseen. One little asteroid, now named 2010 KQ, was picked up by the Catalina Sky Survey on the night of May 16. The fact that this object is small and passing close to Earth is not unusual. Rather it’s orbit is the interesting thing about  this object.  2010 KQ has an orbit that is very Earth-like as the diagram and table below shows.

Orbit of 2010 KQ and the inner planets. Created with C2A. Credit: Carl Hergenrother.

Orbital Parameters:
Perihelion distance = 1.016 AU
Aphelion distance = 1.032 AU
Semi-major axis = 1.024 AU
Inclination = 0.07°
Period = 1.04 years

Currently the official orbit sources give 2010 KQ an absolute magnitude (H) of 28.3 which corresponds to a diameter of 3-8 meters. This H value makes a few assumptions. My independent analysis of it brightness suggests it might be smaller and fainter with an H value of 29.9. This results in smaller diameters of 2-6 meters. Either way this is a small asteroid and only a handful of smaller ones have been detected. I will be able to better define these values as more observations are made.

The very Earth-like orbit creates very slow, long Earth fly-bys. Close approach happened yesterday (May 21) at a distance of 0.0033 AU (~490,000 km or 290,000 miles). That’s just a little further than the Moon.

Small objects on similar orbits have been found in the past. Only one of these objects turned out to be a natural asteroid. All of the others were man-made space hardware. Just like on Earth where it is hard to travel anywhere without running across man-made structures or garbage, space is also littered with working and non-working satellites, rocket bodies, and assorted nuts and bolts.

Bill Gray does an excellent job of monitoring these sorts of objects and determining whether they are natural or man-made. He studies the motion of the objects along their orbits. Man-made objects are light and have a large surface area to mass ratio. This allows the solar wind and other non-gravitational forces to change the motion of the objects. Within a week or so, he should have enough data to declare the object an asteroid or just space junk. He has set up a site where you can follow the current orbit of KQ and his current thoughts on its nature.

The last time KQ was in the vicinity of Earth was back in August 1990. Prior to that it passed Earth in 1975. Perhaps it is a piece of an interplanetary or high-Earth orbit spacecraft mission launched during those times (though the 1990 date already looks unlikely).

Asteroid Fly-By Today

A few hours from now (around 2 hours UT on April 9), a small ~20-meter in diameter asteroid will pass close to the Earth. At that time, asteroid 2010 GA6 will be about 10% closer to Earth than the Moon or 270,000 miles. Even though this is relatively close by asteroid standards, the small object will not be very bright (magnitude 15.5 to 16.0) and will be impossible to see without a camera-equipped telescope.

The asteroid was first picked up by the Tucson-based Catalina Sky Survey early on the evening of April 4. Events of this nature are detected a few times a year and may occur every few weeks. Still there is nothing to worry about. Space is big and these objects are small.

More info and a nice orbit diagram can be found at the JPL NEO Project Office.

In The Sky This Month – February 2010

This feature highlights a number of meteor showers, comets and asteroids which are visible during the month of February 2010. Mars is at its closest and brightest at the beginning of the month. Though it will quickly fade it will remain the dominant planetary body in the evening sky this month.

Note: If anyone has pictures or observations of these objects/events and want to share them, send me a comment and I’ll post them on the blog.

Planets

Venus - Venus is starting its slow climb higher in the evening sky. For the entire month, Venus will only be visible for ~20 minutes after sunset at the start of the month and ~50 minutes after sunset at the end of February. As a result, you will need a clear view of the southwest horizon to see it. Venus will be much higher and easier to see over the next few months. For northern observers, it will be highest in June. The best time for southern observers will be August.

Feb 14 - Moon passes 5° from Venus
Feb 17 - Venus and Jupiter within 0.5° of each other

Jupiter - This month is the last month to see Jupiter in the evening sky. At magnitude -2.0, the King of the Planets is very low in the southwest sky after dusk. By the end of the month it is pretty much invisible to all observers. Observers with a clear SW horizon can watch Jupiter, Venus and the Moon put on a nice show during the middle of the month.

Feb 15 - Moon passes 5° from Jupiter
Feb 17 - Jupiter and Venus within 0.5° of each other

Mars – Mars was at opposition (the point opposite the Sun in the sky) on January 29. Opposition means Mars is closest to Earth and at its brightest. It also means it is visible nearly all night long, rising in east in the early evening, at its highest around midnight, and setting in the west around dawn.

This month the Earth and Mars are slowly moving away from each other. As a result, Mars will quickly fade from magnitude -1.3 to -0.6. Still it will be a brilliant red beacon in the ENE sky right after sundown outshining all but the brightest stars. Note that unlike the stars which twinkle, Mars shines with an unwavering red glow.

Feb 26 - Moon passes close (5°) to Mars

Saturn – Saturn is easy to observe during the morning hours and is sufficiently high enough to be observed in the eastern sky by midnight. Located in Virgo, the planets will appear as bright as a magnitude +0.7 star. Telescope users should note that Saturn’s rings are still close to edge-on.

Feb 2 - Moon and Saturn within 8° of each other

Mercury – Mercury is in the morning sky this month. Northern observers will be able to spot it early in the month as it quickly falls back into the twilight glow. In the south, Mercury starts the month nearly as high as it can get and it should remain visible (with increasing difficulty) for the remainder of the month.

Feb 12 - Moon passes 2° from Mercury

Meteors

February hosts one of the better annual showers of the year in the Quadrantids. Unfortunately this year’s display will be wrecked by bright moonlight. The background rate of meteors crashes in January.  The year is usually split in 2 with January through June having low rates with few major showers while July through December (really through the 1st week of January) have high rates with many major showers.

Sporadic Meteors

Sporadic meteors are not part of any known meteor shower. They represent the background flux of meteors. Except for the few days per year when a major shower is active, most meteors that are observed are Sporadics. This is especially true for meteors observed during the evening. During January, 8-10 or so Sporadic meteors can be observed per hour from a dark moonless sky.

Major Meteor Showers

No major showers this month.

Minor Meteor Showers

Minor showers produce so few meteors that they are hard to notice above the background of regular meteors. Starting this month, info on most of the minor showers will be provided on a weekly basis by Robert Lunsford’s Meteor Activity Outlook.

Additional information on these showers and other minor showers not included here can be found at the following sites: Wayne Hally’s and Mark Davis’s NAMN Notes, and the International Meteor Organization’s 2008 Meteor Shower Calendar.

Comets

Naked Eye Comets (V < 6.0)

None

Binocular Comets (V = 6.0 – 8.0)

None

Small Telescope Comets (V = 8.0 – 10.0)

Comet 81P/Wild 2

Comet Wild 2 is a short-period Jupiter-family comet on a 6.4 year orbit. In 1974 a close approach to Jupiter placed the comet on its current orbit which allows (relatively) close approaches to the Sun and Earth. Swiss professional astronomer Paul Wild found the comet photographically on its first close perihelion in 1978. During its last perihelion passage it was the target of the NASA Stardust spacecraft which flew through its coma, collected cometary dust, and returned the dust to Earth. Though Wild 2 has become bright enough to be seen in small backyard telescopes before, this year’s apparition will be its best since discovery. Not till 2042 will it come closer, and even then only marginally so.

This year Wild 2 will reach perihelion on February 22 at 1.60 AU and closest approach to Earth will occur on April 5 at 0.67 AU. Though the comet will only reach a brightness of magnitude ~9.2 to 9.5 in March, it will remain brighter than magnitude 10.0 from January through May.

Currently the comet is around magnitude 9.5 to 10.0 and should be around magnitude 10.0 or even brighter by the end of the month. At mid-month the comet is located in Virgo (not far from Saturn) at a distance of 1.60 AU from the Sun and 1.86 AU from Earth.

A finder chart for Comet Siding Spring can be found at Comet Chasing and Aktuelle Kometen (in German).

A nice collection of images can be found at the VdS-Fachgruppe Kometen (Comet Section of Germany) and Seiichi Yoshida’s Comet Homepage.

Comet C/2007 Q3 (Siding Spring)

This long-period comet was first seen on 2007 August 25 by Donna Barton of the Siding Spring Survey in Australia. This past Oct. 7th the comet reached a rather distant perihelion at 2.25 AU from the Sun. Unfortunately, the comet and Earth are located on opposite sides of the Sun so the comet is rather far from Earth. Still the comet is observable in the early morning hours as a slowly fading ~9.5 to 10.5 magnitude comet in Bootes. At mid-month the comet is 2.69 AU from the Sun and 2.21 AU from Earth.

A finder chart for Comet Siding Spring can be found at Comet Chasing and Aktuelle Kometen (in German).

A nice collection of images can be found at the VdS-Fachgruppe Kometen (Comet Section of Germany) and Seiichi Yoshida’s Comet Homepage.

Asteroids

Binocular and Small Telescope Asteroids (V < 9.0)

(4) Vesta

Though not as large as Ceres, Vesta is more reflective making it the brightest asteroid in the Main Belt. Vesta is peculiar in that it appears to have evidence of volcanism on its surface. Similar to the Moon, Vesta may be covered with large expanses of frozen lava flows. It is classified as a V-type asteroid and is the only large asteroid with this classification. Many of the smaller V-type asteroids are chips of Vesta blasted off it by past asteroid and comet impacts. Vesta is similar in size to Pallas with dimensions of 347x336x275 miles or 578×560×458 km. Vesta will also be visited by NASA’s Dawn spacecraft which will arrive in 2010.

Vesta starts the month at magnitude 6.4 and brightens to a maximum of 6.1 at opposition on Feb 18. By the end of the month, it has already started to fade at magnitude 6.2. Sixth magnitude is close to the brightest Vesta can get and is easy for binocular observers. If you are lucky enough to be located in a very dark rural site you may even be able to see Vesta by naked eye among the stars of Leo.

A finder chart (needs to be flipped upside down for Northern Hemisphere observers) can be found at the Royal Astronomical Society of New Zealand. Finder chart for Vesta from Heavens Above.

A Few Notes about 2010 AL30

“Mysterious Object Hurtles Towards Earth”

“Weird Object Zooming by Earth Wednesday is Likely an Asteroid”

“Mysterious Space Object Rocks the Web”

The small asteroid 2010 AL30 has made quite a splash in the news today. The titles above are just some examples of news stories floating around the internet. Unfortunately the large number of news stories and blogs calling this object a “mystery” sheds a lot of light on how bad modern reporting can be. As the 3rd headline story above put it: “Something is hurtling toward the planet. And we say ‘something’ because neither we, nor anybody in the scientific field, seems to know exactly what it is.”

Well not quite…

We know exactly what it is. It’s a small asteroid. Sure we don’t know what kind of asteroid it is (is it stoney, basaltic, carbonaceous, iron-nickel?) but those observations will be made soon if they haven’t already. We know the asteroid’s orbit, where it’s been and will be in the future.

The “mystery” part seems to be based on early speculation that AL30 was a returning piece of long-lost space hardware. Such speculation is not crazy and there have been examples of this in the past. An object discovered in 2002 turned out to be the Saturn IVB upper stage from Apollo 12 (I had a small part in confirming its man-made origin). Every time a small asteroid is found on a Earth-like orbit, there is always speculation that it is man-made. Such speculation is usually quickly refuted.

Much of the early speculation of AL30 was posted on the Minor Planet Mailing List (MPML), a Yahoo Groups forum used by amateurs and professionals interested in the observations of asteroids. It is a great forum for sharing ideas and calling attention to interesting observations though the level of expertise ranges from expert to novice. The possibility of AL30 being artificial was quickly brought up on the MPML and then just as quickly refuted. Unfortunately that short exchange was enough to get the asteroid labeled as “mysterious”. Once one trusted news source calls it a “mystery” it isn’t long before the average “copy and paste” stories spread like wild fire.

Even if AL30 was man-made, it really wouldn’t be much of a mystery. We’d quickly figure out what launch it was a part of. It’s orbit is now well determined allowing the NEO Project Office at JPL to find no instances of a previous close approach to Earth in the past 50 years. This rules out a man-made origin. Then again calling it a “mystery” in the press may get more people to read the story. Science can be boring if we have all the answers. Maybe by making it seem that we are uncertain of something, it makes it appear that we can’t rule out anything (maybe its an asteroid, a comet, a man-made spacecraft, maybe even something secret, or an UFO) and that makes for a better story.

Enough ranting, what do we actually know about 2010 AL30…

Not much has changed from my previous posting on the object. It is still predicted to pass within 129,000 km of Earth tomorrow morning (Jan 13) at ~12:46 UT.  The figure below by JPL shows its path through cislunar space.

Path of 2010 AL30 on Jan 13. Credit: NASA/JPL.

What if AL30 were to hit the Earth? How much of an impact (no pun intended) would it make?

A group at the University of Arizona/Lunar and Planetary Lab has created a web GUI (called the Earth Impact Effects Program) that estimates the effect of an impacting asteroid on Earth. Entering in the following parameters for AL30 (these are just estimates but are realistic based on our current knowledge of the object):

Inputs:

• Projectile Diameter: 15.00 m = 49.20 ft = 0.01 miles
• Projectile Density: 3000 kg/m³
• Impact Velocity: 9.50 km/s = 5.90 miles/s
• Impact Angle: 45 degrees
• Target Density: 2500 kg/m³

gives us the following results …

Energy:

• Energy before atmospheric entry: 2.39 x 10¹⁴ Joules = 0.57 x 10^-1 MegaTons TNT
• The average interval between impacts of this size somewhere on Earth is 12.1 years

Atmospheric Entry:

• The projectile begins to breakup at an altitude of 44700 meters = 147000 ft
• The projectile bursts into a cloud of fragments at an altitude of 22900 meters = 75100 ft
• The residual velocity of the projectile fragments after the burst is 6.12 km/s = 3.8 miles/s
• The energy of the airburst is 1.40 x 10¹⁴ Joules = 0.33 x 10^-1 MegaTons.
• No crater is formed, although large fragments may strike the surface.

This would be one nice fireball and look very similar to many of the bright ones reported on this blog. Small pieces of the asteroid would probably survive to reach the ground as meteorites. Note, this group states that a 15-meter asteroid should hit the Earth once every 12 years while in yesterday’s posting I said once every 50 years. The once every 50 years number comes from global infrasound data and the every 12 year interval from asteroid survey data. It is probably safe to say that a 15-meter impact happens once every 10-50 years.

One other caveat, the above calculation assumes the asteroid is a fractured rock. If it were a solid piece of nickel-iron, the impact would be much greater as seen below.

Atmospheric Entry:

• The projectile begins to breakup at an altitude of 3470 meters = 11400 ft
• The projectile reaches the ground in a broken condition. The mass of projectile strikes the surface at velocity 5.65 km/s = 3.51 miles/s
• The impact energy is 2.26 x 10¹⁴ Joules = 0.54 x 10^-1MegaTons.
• The broken projectile fragments strike the ground in an ellipse of dimension 0.101 km by 0.0714 km

Crater Dimensions:

• Crater shape is normal in spite of atmospheric crushing; fragments are not significantly dispersed.
• Final Crater Diameter: 427 m = 1400 ft
• Final Crater Depth: 91 m = 299 ft
• Richter Scale Magnitude: 3.7

Luckily nickel-iron meteorite falls are very rare and a very minor fraction of all impactors.

Remember to Duck Wednesday Morning

A very small, newly discovered asteroid will make a close flyby of Earth this Wednesday. 2010 AL30 was first seen by the LINEAR survey (same group that discovered the unusual comet P/2010 A2) on Jan 10 UT. The asteroid will pass ~80,000 miles or 128,000 km (0.00086 AU) from Earth at ~12:45 UT on Jan 13 (Wednesday morning).

With an absolute magnitude of 27.0, the asteroid is probably on the order of 10 to 20 meters in diameter. Though it will safely miss the Earth, even if it were to hit it would be unlikely to do any damage as very little of it would survive passage through the atmosphere. A recent paper by Peter Brown (University of Western Ontario) determined the rate of impact for small asteroids based on global infrasound observations. An object 15 meters in diameter should hit the Earth once every ~50 years. Based on that statistic the detection of an object like 2010 AL30 should be rare. But the Earth is a small target. Extrapolating the Earth collisional volume to the volume of space within the distance of the Moon finds that ~600 objects of the size of 2010 AL30 pass within a lunar distance every year. These kinds of close approaches are not rare, and the great majority of them are missed by the current asteroid surveys which are optimized to find much larger (and actually dangerous) objects.

The orbit of 2010 AL30 is a typical near-Earth asteroid orbit (see the diagram below). It stretches from 0.70 to 1.30 AU from the Sun. With a semi-major axis of 1.001 AU, its orbital period is almost exactly the same as Earth’s. At its brightest the asteroid will reach magnitude ~14 which is far too faint to be seen by all but the most well equipped amateurs.

Orbits and positions of 2010 AL30 and the inner planets for 2010 Jan 13. Created with C2A.

x

More on the Jupiter Impact

Two nights ago an Australian amateur astronomer, Anthony Wesley, observed a dark spot in the South Polar region of Jupiter. He quickly realized that this feature was out-of-the-ordinary and notified the astronomical community. His announcement allowed professional astronomers with access to some of the largest and best telescopes in the world to observe the new spot. So far the consensus points to a recent impact by a comet or asteroid as the cause of the spot.

Amateur astronomers around the world have been able to observe and image the spot. In most images it is not obvious what the spot is. Luckily Anthony Wesley has an excellent set-up for planetary imaging and was able to recognize the spot as something unusual.

Bob Lunsford (San Diego, CA) was able to image the Jupiter and its new spot with his 9.25″ SCT telescope. The image nicely shows the impact spot. It is located near the upper right limb of Jupiter. Though hard to see, the large but pale circle near the upper left limb of Jupiter is its famous Great Red Spot. These days the Great Red Spot is still great but not as red as it once was.

Jupiter and its new dark impact spot as imaged by Bob Lunsford with his 9.25" SCT on 2009 July 21 at 1106 UT. Image credit: Bob Lunsford.

Professionals on the Keck 10-meter telescope and NASA’s Infrared Telescope Facility (IRTF) were able to observe the spot. According to Keck observations published on Central Bureau Electronic Telegram 1882, the spot covers 200 million square kilometers which is nearly half of the Earth’s surface area. The impact site consists of 2 feature located 2 degrees apart and an ejecta field 10 degrees long. The image below is from a NASA press release on observations taken with the IRTF in Hawaii.

This image shows a large impact on Jupiter's south polar region captured on July 20, 2009, by NASA's Infrared Telescope Facility in Mauna Kea, Hawaii. Image credit: NASA/JPL/Infrared Telescope Facility.

How do we know that this spot was caused by an impact?

Back in 1993, astronomers Carolyn and Gene Shoemaker and David Levy discovered one of the most unusual comets ever seen. Rather than the usual “puffball” with a tail, Comet Shoemaker-Levy 9 appeared to be a fuzzy bar. Observations with larger telescopes revealed a string of individual comets all traveling in roughly the same orbit. Further observations, like the Spacewatch image below,  helped uncover the origin of this weird object.

Comet P/Shoemaker-Levy 9 as it appeared on 1993 March 30, a few days after discovery. This image was obtained with the Spacewatch telescope. Image credit: Jim Scotti, Spacewatch, University of Arizona.

Comet Shoemaker-Levy 9 had been captured onto a temporary orbit around Jupiter. Some studies found that it may have been orbiting Jupiter for ~20 years before it was discovered. Its orbit about Jupiter was very elongated and took many months to complete one circuit. In July of 1992  the comet passed so close to Jupiter that Jupiter’s gravity ripped it into pieces. When the comet was first seen by the Shoemaker-Levy team, it was still in dozens of pieces and still in orbit around Jupiter.

Gravitational perturbations caused the comet’s orbit to change to the extent that the comet would not only come close to Jupiter at its next swing-by but would actually hit the planet. Between the days of July 16 and 22, 1994, the many pieces of Shoemaker-Levy 9 impacted the planet. Each impact left a brownish scar on the upper atmosphere of Jupiter. The scars are the result of the fireball caused by the impacting comets pulling material up from deep within Jupiter’s atmosphere. The Hubble Space Telescope picture below shows some of the marks left by SL9 in 1994.

Brown spots mark the places where fragments of Comet Shoemaker-Levy 9 tore through Jupiter's atmosphere in July 1994. Credit: Hubble Space Telescope Comet Team and NASA.

Because we have seen impacts on Jupiter before, it is much easier to recognize impacts today. It would be interesting to see what astronomers would think of today’s dark spot if we hadn’t witnessed the impact of SL9. Would we even take much notice?

So what was it that hit Jupiter (and how did we miss it)?

Based on the SL9 impact scars, the recent impactor was probably on the order of a kilometer or so across. We don’t know if it was a comet or asteroid though I suspect a thorough analysis of the large telescope data may reveal the answer. At the distance of Jupiter, most of the objects on stable orbits are asteroids (or old comets that are no longer active) while the objects that are on unstable orbits are comets. In order to impact Jupiter, an object must be on an unstable orbit so it is likely that it was a comet.

At the distance of Jupiter (5.2 AU or 5.2 times further from the Sun than Earth is) few comets are active. In the case of SL9, the comet was active due to its very close approach to Jupiter. Assuming a small, dark, inactive 1-km comet nucleus, it would have been no brighter than 25th magnitude. Even today’s advanced asteroid survey telescopes have trouble going much fainter than 22nd magnitude which is 16 times brighter than 25th magnitude. It is very likely that the impactor was simply too faint to be detected by the surveys. Many of the largest telescopes around the world, as well as the Hubble Space Telescope, can see faint enough to have discovered the impactor. But… these scopes have such small fields of view that it is impractical to use them to survey the sky for new objects. So we may never know exactly what hit Jupiter.

[In case your wondering, a 1-km wide asteroid orbiting near Earth is much brighter and easier to find. In fact, it is estimated that we have already found close to 90% of the 1-km asteroids and comets that threaten Earth. With more capable asteroid surveys coming online in the next 5 years or so we may find nearly all of the large Earth threatening asteroids over the next 2 decades.]

There is little chance that the recent impactor is related to SL9 even though they impacted almost exactly 15 years apart. The exactly 15 year interval is a fluke since Earth years mean nothing to Jupiter. It takes Jupiter just under 12 years to orbit the Sun so Jupiter is located in a different part of its orbit than it was in 1994. The 15 year interval does tell us that Jupiter impacts may happen often. Right after SL9 a study found that a Jupiter impact should happen once every 20 years or so.